<i>In Situ</i> Monitoring the Nucleation\nand Growth of Nanoscale CaCO<sub>3</sub> at the Oil–Water Interface

Interfaces can actively control the\nnucleation kinetics,\norientations,\nand polymorphs of calcium carbonate (CaCO<sub>3</sub>). Prior studies\nhave revealed that CaCO<sub>3</sub> formation can be affected by the\ninterplay between chemical functional moieties on solid–liquid\nor air–liquid interfaces as well as CaCO<sub>3</sub>’s\nprecursors and facets. Yet little is known about the roles of a liquid–liquid\ninterface, specifically an oil–liquid interface, in directing\nCaCO<sub>3</sub> mineralization which are common in natural and engineered\nsystems. Here, by using <i>in situ</i> X-ray scattering\ntechniques to locate a meniscus formed between water and a representative\noil, isooctane, we successfully monitored CaCO<sub>3</sub> formation\nat the pliable isooctane–water interface and systematically\ninvestigated the pivotal roles of the interface in the formation of\nCaCO<sub>3</sub> (i.e., particle size, its spatial distribution with\nrespect to the interface, and its mineral phase). Different from bulk\nsolution, ∼5 nm CaCO<sub>3</sub> nanoparticles form at the\nisooctane–water interface. They stably exist for a long time\n(36 h), which can result from interface-stabilized dehydrated prenucleation\nclusters of CaCO<sub>3</sub>. There is a clear tendency for enhanced\namounts and faster crystallization of CaCO<sub>3</sub> at locations\ncloser to isooctane, which is attributed to a higher pH and an easier\ndehydration environment created by the interface and oil. Our study\nprovides insights into CaCO<sub>3</sub> nucleation at an oil–water\ninterface, which can deepen our understanding of pliable interfaces\ninteracting with CaCO<sub>3</sub> and benefit mineral scaling control\nduring energy-related subsurface operation.